Apparatus for clarifying an effluent, known as a clarifier, enables:                clarification of the effluents,        separation of the purifying biomass and the most dense particles of the treated effluent by gravity settling,        thickening, that is to say densification, of this sludge,        collection and retention of sludge which must be extracted:                    and recirculated upstream in the treatment line,            and/or evacuated to the sludge treatment and/or evacuation line.                        
The collection and the extraction of the sludge from the bottom of the structure in a limited time is of great economic and technical importance in clarifiers.
In fact, if the extraction of sludge is insufficient and/or unevenly effective over the whole of the surface of the structure, sludge accumulates on the bottom of the structure; portions of this sludge are deprived of oxygen (prolonged anoxia), which causes their physical, mechanical and biological deterioration and as a consequence of this serious disturbances throughout the treatment line, such as:                deterioration of the quality of the treated water: increased contents of materials in suspension and phosphorus,        degraded settling of the sludge and therefore the treatment capacity of the clarifier, i.e. its technical/economical effectiveness,        degraded purification quality of the sludge and therefore the effectiveness of treatment by the whole of the purification line,        degraded treatability of the sludge,all of which has an economic impact on the treatment cost.        
Moreover, the accumulation of sludge on the bottom of the structure leads to high mechanical stresses on the sludge take-up plant and underlying reliability problems in respect of this plant, which:                reduces the availability of the structures, and        increases maintenance (labor) costs and plant renewal costs.        
A contrario, because the extracted flow of sludge is:                recirculated to the treatment line, including the clarifier itself, it must be minimized in order to limit:                    the mass load per unit surface area of the clarifier (kg of dry materials/m2·h),            the sludge extraction plant investment costs (size of extraction and recirculation pumps) and operating costs,                        and/or evacuated to the sludge treatment unit, it must be minimized in order to optimize the sizing and the technical/economic operation of the sludge line.        
The efficacy, ruggedness and reliability of the device for taking up sludge from the bottom of the structure are therefore of great importance in sludge extraction:                necessary to limit the residence time of the sludge in the structure, and        sufficient not to increase the hydraulic and mass loads on the structures and the treatment plant of the water line and/or the sludge evacuation and treatment line.        
The clarification apparatus at which the invention is aimed, also known as “sucked” clarifiers, is well suited to large clarification structures, that is to say to structures in which the basins, in theory circular, have a diameter that is generally greater than 25 m. The suction tubes are immersed directly in the bed of the sludge that has settled on the bottom of the structure and sweep the whole of the bottom surface. These tubes open into the sludge take-up trough on the surface, the hydraulic level of which is kept lower than that of the clarifier.
To enable individual adjustment of the suction rate of each of the tubes, each tube is provided with a fixed but adjustable telescopic sleeve at the overspill level. This sleeve is installed at the outlet of each suction tube and the liquid and the sludge overspill over the upper end of the telescopic sleeve. This difference between the liquid level in the clarifier and the liquid level at the outlet at the top of the sludge overspill telescopic tubes in the trough provides the motive force for the suction and therefore extraction of the sludge. The adjustment of the position of each sleeve must ensure evacuation of the sludge at the bottom of the structure such that there is no accumulation and the bed of sludge has substantially the same thickness over all of the surface of the bottom of the structure.
In the prior art, the level of the upper edge of the sleeves is adjusted by means of chains or threaded rods or notched rods to which the sleeves are attached, the chains or rods being themselves attached to a support.
The adjustment of the levels of each sleeve requires:                a laborious relatively long iterative manual process of trial and error, taking of the order of one day for a structure having a diameter equal to or greater than 25 m, aiming to measure and to obtain a necessary and sufficient sludge extraction mass flow rate for each of the tubes,        human intervention and manipulation on a turning bridge generally constituting the mobile arm, with risks of falling in that must be limited for reasons of the safety of the operators and the plant.        
The flow rates of sludge to be evacuated are variable, notably in the case of purification station sludge, depending on atmospheric constraints, notably in the event of storms. Under other conditions, however, the flow rate of sludge to be evacuated can be much lower.
In practice, in prior art clarifiers, the levels of the sleeves are adjusted for the highest flow rate to be evacuated. The frequency of adjustment of the levels of the sleeves is limited and in no case is the level of the sleeves adjusted continuously and/or in real time to suit the real requirements of the station.
The liquid level of the sludge in the take-up trough must be maintained:                below the liquid level in the clarifier to enable extraction of the sludge by the motive force aspirating the sludge caused by the difference in levels; and        below the outlet levels of the suction tubes, meaning that the tube must necessarily emerge relative to the liquid level in the trough, so as to be able to balance the flow rates through each suction tube in order to obtain a good distribution of the extracted mass flows between the suction tubes; the flow rate through each suction tube is inversely proportional to the viscosity of the fluid conveyed and therefore the sludge concentration.        
The differences in hydraulic levels are maintained thanks to permanent external pumping to evacuate sludge from the trough, constituting the means for evacuation of this sludge.
The upper end of the sleeves of the suction tubes should therefore be, at one and the same time:                continuously sufficiently high to emerge from the liquid level in the trough regardless of the flow rate at which the station is operating, and        as low as possible relative to the upper hydraulic level in the basin of the clarifier to ensure a maximum flow rate through the suction tubes and to enable extraction of the necessary and sufficient masses of sludge when the station is functioning at its maximum hydraulic and/or mass capacity.        
To satisfy these two antagonistic constraints, the outlet level of the sleeves of the suction tubes being in theory adjusted once and for all, and not continuously adjustable, this requires the pumping flow rate in the trough to be permanently as high as possible so that the adjustment of the sleeves of the tubes is correct, that is to say with the sleeves emerging from the water during rain and/or when the station is operating with the maximum flow rate or load.
Depending on these adjustments, and the resulting operation of the apparatus, the sludge extraction and recirculation flow rate is maintained constant and maximum, with no possibility of regulation, irrespective of the real hydraulic conditions under which the station is operating and the real sludge extraction requirements.
Depending on these adjustments, and on the resulting operation, the sludge extraction and recirculation flow rate is, in fact, too high during periods of low hydraulic and/or mass flow rate in the operation of the station (nocturnal periods, periods of dry weather, less polluted effluents leading to lower production of sludge), causing in the water line:                overconsumption of pumping energy (by a recirculation pump),        unnecessary wear of the pumping plant with consequences for maintenance costs and plant renewal frequency,        mass hydraulic overloads on the clarifier that can lead to the entrainment of materials in suspension in the clarified treated water, i.e. deterioration in the quality of the treated water,        an increased risk of entrainment of air bubbles and therefore of unpriming of the device for evacuating sludge from the trough.        
Depending on these adjustments, and the resulting operation, the concentration of the extracted sludge is, in fact, lower than necessary most of the time, and the volumes of sludge to be evacuated and treated are therefore unnecessarily higher than necessary in periods of low hydraulic and mass flow rates in the operation of the station, the consequence of which is to induce in the sludge line:                overconsumption of pumping energy,        hydraulic overloads of the sludge line (as a result of degraded operating conditions), and/or        increased operating times of the treatment unit, increasing electrical power consumption, plant wear and labor costs,        substantial increases in consumption of reagents,        possible reduction of the dry content of the treated sludge and therefore the volumes of sludge treated, which leads to an increase in treated sludge handling and evacuation/management costs.        
Moreover, in the prior art, despite the usual precautions in respect of the adjustment of the sleeves, the risk of totally immersing some or all of the suction tubes if the level in the take-up trough rises above the overspill level of the sleeves is not controlled in the case of:                exceptional hydraulic flow rates,        malfunctioning and/or stopping of the pump for evacuation of sludge from the trough, which can lead to the accumulation of fibrous waste and chaff at the level of the devices for adjustment of each of the telescopic sleeves equipping the suction tubes, with resulting risks in respect of the service life of the plant and the need for time-consuming cleaning operations that are hazardous for personnel and plant.        